Add PowerShell jobs support

[daviwil]

The PowerShellContext should be able to run PowerShell jobs without issue. A user of the VS Code extension has reported that they're not able to run a script that starts a job. More info here: PowerShell/vscode-powershell#120

[daviwil]

Reopening, I think the vscode-powershell bug number slipped into the commit :)

[rkeithhill]

Yeah, sorry about. I originally put the VSCode issue number in the commit message for 240 and started the PR but I changed the comment to the correct issue number before submitting the PR. I was hoping that would correct things but apparently not. :-(

[daviwil]

No problem at all, man. This is the problem with having two repos for one effective product ;)

[TylerLeonhardt]

I believe this is fixed now. However, there's one open comment in the other issue we need to hear about.